Maleimide copolymer as alignment layer for liquid-crystal displays

ABSTRACT

Use of a polymer containing maleimide units of the formula I ##STR1## in which R 1  is hydrogen, an acyclic or cyclic, aliphatic, aromatic or araliphatic radical which is chiral or achiral, can be monosubstituted or polysubstituted by functional groups and in which one or more CH 2  groups can be replaced by functional groups, as alignment layer in liquid-crystal displays. 
     Alignment layers containing a polymer conforming to the formula I effect, in particular, a suppression of twist states and ghost images and thus an improvement in the optical contrast.

BACKGROUND OF THE INVENTION

Switching and display devices containing ferroelectric liquid-crystalmixtures (FLC displays) are disclosed, for example, in EP-B 0 032 362(U.S. Pat. No. 4,367,924). Liquid-crystal displays are devices which, asa consequence of electrical switching, modify their optical transmissionproperties in such a way that incident (and possibly re-reflected) lightis modulated in intensity. Examples are the known watch and calculatordisplays or liquid-crystal displays in the OA (office automation) or TVsectors (see also Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun,Tokyo, 1989; ISBN 4-526-02590-9C 3054 and the papers cited therein).

These FLC displays are constructed in such a way that a ferroelectricliquid-crystal layer is enclosed on both sides by layers which areusually, in this sequence starting from the FLC layer, at least onealignment layer, electrodes and a limiting plate (for example made ofglass). In addition, they contain one polarizer if they are operated inguest-host or reflective mode or two polarizers if the transmissivebirefringence mode is used. The switching and display elements maycontain further auxiliary layers, such as diffusion barrier orinsulation layers.

The alignment layers, which comprise an organic (for example polyimide,polyamide and polyvinyl alcohol) or inorganic (for example SiO)material, together with a separation between the limiting plates whichis chosen to be sufficiently small, bring the FLC molecules of themixture into a configuration in which the molecules lie with their longaxes parallel to one another and the smectic planes are arrangedperpendicular or inclined to the alignment layer. In this arrangement,the molecules, as is known, have two equivalent orientations, betweenwhich they can be switched by pulse-like application of an electricfield, i.e. FLC displays are capable of bistable switching. Responsetimes are inversely proportional to the spontaneous polarization of theFLC mixture and are in the region of microseconds.

Surprisingly, it has now been found that polymers containing maleimidemonomer units can advantageously be employed as alignment layers inliquid-crystal displays, in particular ferroelectric displays.

Polymers of this type have hitherto been employed as binders inphotoresist materials, as described, for example, in U.S. Pat. No.720,445, EP 0 140 273 and EP 0 234 327. However, their use as analignment layer in LC displays, in particular FLC displays, has notpreviously been described.

SUMMARY OF THE INVENTION

The invention thus relates to the use of a polymer containing maleimideunits of the formula I ##STR2## in which R¹ is hydrogen, an acyclic orcyclic, aliphatic, aromatic or araliphatic radical which is chiral orachiral, can be monosubstituted or polysubstituted by functional groupsand in which one or more CH₂ groups can be replaced by functionalgroups,

as an alignment layer in liquid-crystal displays, preferably inferroelectric liquid-crystal displays.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

R¹ is preferably hydrogen, an aromatic, aliphatic or araliphatic ringhaving 6 to 24 carbon atoms or a branched or unbranched aliphaticradical having 1 to 40 carbon atoms, which may also contain chiralcenters and in which one or more hydrogen atoms, independently of oneanother, may be replaced by --OH, --F, --Cl, --Br, --CN, --NR² R³,--COOR², --OR², --OSi(CH₃)₃, --SiR² ₂ R³, --Si(OR²)₂ R³, --Si(OR²)₂(OR³) or --OOC--NR² R³, where R² and R³, independently of one another,are hydrogen or an alkyl radical having 1 to 6 carbon atoms, and inwhich one or more CH₂ groups may be replaced by --O--, --SO₂ --, --CO--,--CONR² --, --HC═CH-- or --C.tbd.C--.

R¹ is particularly preferably hydrogen, a branched or unbranched alkylradical having 1 to 20 carbon atoms in which one CH₂ group may bereplaced by --O-- or --CO--, and in which one or more hydrogen atoms maybe replaced by fluorine, or is --CH(CH₃)--CH₂ --(O--CH₂ --CH₂ --)_(n) X,--CH₂ --CH₂ --(O--CH₂ --CH₂)_(n) X, --CH₂ (CH₃)--CH₂ --(O--CH₂--CH(CH₃)--)_(n) X or --CH₂ --CH₂ (O--CH₂ --CH(CH₃)--)_(n) X, where n=1to 10 and X=--NH₂ or --OH, or the ##STR3## The maleimide units of theformula I are preferably in the main chain.

The polymer to be employed according to the invention can comprise, as ahomopolymer, only maleimide units of the formula I having identical R¹radicals or, as a copolymer or higher polymer, maleimide units of theformula I containing different radicals R¹ or maleimide units of theformula I and units derived from polymerizable, ethylenicallyunsaturated compounds of the formula II ##STR4## in which R⁴ to R⁷ arehydrogen, or aliphatic and/or aromatic radicals, which can bemonosubstituted or polysubstituted by functional groups.

R⁴ to R⁷ are preferably hydrogen, an aromatic radical having 6 to 10carbon atoms or a branched or unbranched alkyl group having 1 to 10carbon atoms, in which one or more non-adjacent CH₂ groups may bereplaced by --O--, --OOC--, --COO--, --Si(CH₃)₂ -- or --O--CO--NR⁸, orare --OH, --Cl, --Br, --NO₂, --CN, --COOR⁹, --OR¹⁰, --O--Si(CH₃)₃ or--O--CONR¹¹ R¹², where R⁸, R⁹, R¹⁰, R¹¹ and R¹² are hydrogen or an alkylradical having from 1 to 5 carbon atoms.

Particularly preferred olefinic monomers of the formula II are styrenes,vinyl and allyl ethers, vinyl and allyl esters, vinyl- orallyltrimethylsilane, acrylonitrile, cinnamic esters and cinnamonitrile,acrylic and methacrylic esters, acrylamides, methacrylamides andvinylnaphthalenes.

Particularly preferred olefinic monomers are styrenes of the formula IIIand/or vinyl ethers/esters of the formula IV ##STR5## in which R¹³ ishydrogen or methyl, and

R¹⁴ and R¹⁵ are hydrogen, acyl having 1 to 5 carbon atoms, --OH, --Cl,--Br, --F, --CN, --NO₂, --NR¹⁷ R¹⁸, --COOR¹⁹, --OOC--R²⁰ or --O--COOR²¹,

where R¹⁷, R¹⁸, R¹⁹, R²⁰ and R²¹ are hydrogen or an alkyl radical having1 to 5 carbon atoms, in which one or more hydrogen atoms may besubstituted by --OH, --Cl, --Br, --F, --CN or --NO₂, and

R¹⁶ is alkyl having 1 to 30 carbon atoms, in which one or more H atomsmay be substituted by --OH, --Cl, --Br, --F, --CN or --NO₂, acyl having1 to 5 carbon atoms or an aromatic group having 6 to 10 carbon atoms.

Polymers comprising units of the formulae I and II contain at least 5%,preferably from 25 to 75%, particularly preferably from 40 to 60%, ofmaleimide units of the formula I.

The polymers to be employed according to the invention are particularlyadvantageously copolymers or terpolymers comprising 50% of maleimideunits of the formula I and a total of 50% of units of a styrene of theformula III and/or units of a vinyl ether of the formula IV, where R¹ ishydrogen, --CH₂ OH or --C₆ H₄ --OH, R¹⁰ and R¹¹ are hydrogen, R¹² ishydrogen or C₁ -C₆ -alkyl, and R¹³ is C₆ -C₂₄ -alkyl. The ratio betweenthe number of units of the formulae III and IV is preferably from 100:0to 0:100, particularly preferably from 100:0 to 50:50.

It is particularly advantageous to employ a polymer of the formula Vbelow, ##STR6##

The polymers to be employed according to the invention, in particular ofthe formula V, can have a molecular weight (M_(w)) of from 3000 to300,000, preferably from 5000 to 100,000, particularly preferably from10,000 to 50,000. The molecular weight is determined by GPC (gelpermeation chromatography).

The polymer can be prepared in a manner known per se by free-radicalpolymerization using AIBN or DBPO as free-radical initiator, inconcentrations of from 1 to 5 mol %.

It is possible to vary the radical R¹ on the imide nitrogen in variousways. For example, it is possible, as described above, to start frommonomers which already contain the desired radical R¹.

It is also possible subsequently to modify the maleimide component inthe copolymers and terpolymers listed here. This can be effected, forexample, by deprotonating the imide (if R¹ =H) by means of a suitablebase and subsequently reacting the product with an acyl halide. Thistype of reaction is known per se and described, for example, inOrganikum, 15th Edn., p. 257, VEB Dt. Vlg. d. Wissenschaften, Berlin,1981. During the reaction of the deprotonated imide with acyl halides,rearrangement reactions can occur, in particular under the conditionsfor the production of the alignment layer, so that the structure of thenovel compound in the finished liquid-crystal cell need not necessarilybe identical to the primary product of the above reaction.

The imide component can furthermore be modified under the conditions ofthe Mitsunobu reaction (O. Mitsunobu, Synthesis, 1981, 1). In thisreaction, a novel maleimide copolymer is reacted with an alcohol in thepresence of stoichiometric amounts of triphenylphosphine and diethylazodicarboxylate with elimination of water. If optically active alcoholsare used, the chirality is retained during the reaction, so thatchirally modified alignment layers are obtained in this way.

It is also possible subsequently to introduce the maleimide componentclaimed into the copolymer or terpolymer mentioned here by starting froma copolymer or terpolymer containing a maleic anhydride in place ofmaleimide. This polymer can then, in a second step, be converted intothe maleimide copolymer claimed here in a known manner (Houben-Weyl,Vol. 8/p. 657, G. Thieme Verlag, 1952) by reaction with ammonia orprimary amines (R₁ --NH₂) with removal of the water liberated during thereaction. In this case, it is unnecessary to achieve complete conversionof the maleic anhydride units into maleimide units, since a residualcontent of maleic anhydride in the polymer does not interfere. However,it is advantageous to convert at least 30%, preferably 50%, of theanhydride groups.

It is furthermore possible to start from a commercially availablecopolymer comprising only maleic anhydride and compounds of the formulaII and, after application, to convert the resultant alignment layer intomaleimide units only at the surface by reaction with a primary amine. Inthis case, it is also possible to carry out the imidation usingdiamines, preferably relatively long-chain and/or terminal diamines,which result in crosslinking or loop formation.

It is furthermore possible to use chiral compounds, for example aminoacid esters, where the chirality is also retained in this procedure.Chiral alignment layers of this type enable a particularly high degreeof order to be obtained. If the surface modification is carried outusing amines which contain at least one further amino group and acarboxyl, sulfonyl or phosphonyl group and which therefore formzwitterions, ionically modified alignment layers are obtained. Examplesof such compounds are lysine, arginine, α, β-diaminobutyric acid,ornithine, hydroxylysine and/or citrulline.

All the mono-, di- or polyamino compounds mentioned can be employedindividually or as a mixture.

Use may be made according to the invention of homo-polymers whichcomprise only maleimide units of the formula I containing identical R¹radicals or of copolymers and higher polymers comprising maleimide unitsof the formula I containing different radicals R¹ or comprisingmaleimide units of the formula I and units derived from polymerizable,ethylenically unsaturated compounds of the formula II, which aredescribed above.

The invention furthermore relates to a polymer containing maleimideunits of the formula I which has been rendered amphiphilic, and to theuse thereof as alignment layer in liquid-crystal displays, preferably inferroelectric liquid-crystal displays.

Polymers which have been rendered amphiphilic are prepared by applyingor linking by covalent bond compounds, for example, of the coronand,cryptand or podand type. The polymer which has been rendered amphiphiliccan be employed as alignment layer and results in significantlyincreased contrast in FLC displays and in greater image brightness.

The substances causing the amphiphilicity can either be chemicallybonded to the alignment layer or applied simply as a strongly or weaklyphysisorbed coating.

The display component causing the amphiphilicity of the alignment layercan thus be applied as an additional layer between the alignment layerand the FLC layer or added to the material of the alignment layer as asimple mixture component. A further possibility comprises coupling thesubstance to the material of the alignment layer by chemical reaction.

The effective intermediate layer can be applied, for example, from asolution of suitable compounds in acetone, toluene, cyclohexanone,isopropanol, N-methyl-pyrrolidone, dioxane or the like by printing,dipping, spraying and spin-coating methods or the like. Also suitableare vacuum deposition methods, such as simple vapor deposition orreactive vapor-deposition methods (for example chemical vapor deposition(CVD)).

The effective intermediate layer can be applied during various steps inthe FLC display production process, for example directly after thecuring or drying of the alignment layer, before the rubbing step orimmediately before the bonding of the cell. The substances or substancemixtures can likewise be applied to the wet alignment layer film andcured, i.e. heated, simultaneously with the alignment layer.

It is also advantageous to mix the active substances or substancemixtures with the polymer or polymer precursor solution prepared for theproduction of the alignment layer and then to apply them together withthe latter in one step.

The active compounds can in principle be either monomeric, oligomeric orpolymeric compounds. In general, they have a moderate to stronglipophilic character with low polarity, or are distinguished by the factthat the compound has separately localized regions of relatively highand low polarity/hydrophilicity. Cyclic compounds can also have anexosphere having a rather lipophilic nature and an endosphere having arather hydrophilic nature.

It is furthermore preferred, in LC displays containing the novelalignment layer, to use insulation layers in order to avoid shortcircuits, where the layer sequence is

(1) glass,

(2) ITO electrode,

(3) insulation layer,

(4) alignment layer, if desired with additive admixed or chemically orphysically bonded to the surface.

In order to suppress the surface memory effect, it may be advantageousfor the electrical capacitance of the insulation and aligrunent layersto be as high as possible (in this respect, cf. C. Escher, H. -R. Dubal,T. Harada, G. Illian, M. Murakami and D. Ohlendorf, 2nd Int., Symp. onFLC, Gothenburg, 1989, Ferroelectrics 113 (1991) 269.

For adequate insulation capacity, the thickness of the insulation layershould be at least from 50 to 100 nm. In order to achieve sufficientlyhigh capacitances at this layer thickness, insulation layers having highdielectric constants, such as Ta₂ O₅ and TiO₂, must be used (see alsoJP-A 61/170 726, JP-A 61/078 235, Y. Inabe, K. Katagiri, H. Inone, J.Kanbe, S. Yoshihara and S. Iijima, Ferroelectrics (1988), 85, pp. 255 to264).

Particularly suitable compounds which render the alignment layeremphiphilic are macrocyclic compounds, cryptands, coronands, podands,mercapto compounds and ionophoric compounds.

Macrocyclic compounds to be employed according to the invention aredescribed in EP-A-0 451 822 and are reproduced here by means of theformula VI ##STR7## where a, b, c, d, e and f, independently of oneanother, are integers from zero to 4, where a+b+c+d+e+f is ≧7, and

-A-, -B-, -C-, -D-, -E- and -F- are identical or different and are##STR8## where R is alkyl having 1 to 12 carbon atoms, and

R' is alkyl having 1 to 12 carbon atoms in which one --CH₂ -- group maybe replaced by --O-- or --CO O--, or is phenyl or Cl, F or CN.

Cryptands and coronands, as proposed in DE-A 4 011 803, are likewiseparticularly suitable as compounds providing a amphiphilic effect.

For a classification of said complex ligands, reference is made to E.Weber and F. Vogtle, Inorganica Chimica Acta, Vol. 45, (1989), L65-L67.The ligand topo-logies listed therein are reproduced below: ##STR9##

The cryptands or coronands to be employed can be represented by theformula VII or VIII: ##STR10## where Z is --O-- or --S--,

m and n are integers greater than zero, where m+n=2 to 6,

--X¹ -- and --X² -- are identical or different and are ##STR11## or --X¹-- and --X² -- together are

>N--CH₂ (--CH₂ --Z--CH₂)_(t) --CH₂ --N< or

>N--CO(--CH₂ --Z--CH₂)_(t) --CO--N<

where

--R is alkyl or alkanoyl having 1 to 15 carbon atoms, phenyl, benzyl orbenzoyl, and

t is 1 or 2; ##STR12## where --R¹, --R², --R³ and --R⁴, independently ofone another, are ##STR13## and p, q, r and s, independently of oneanother, are an integer from 2 to 4, where p+q+r+s=8 to 16.

Preferred coronands are: ##STR14##

Mercapto compounds to be employed according to the invention arerepresented by the formula IX below: ##STR15## where R¹, R², R³ and R⁴,independently of one another, are a hydrogen atom, alkyl having 1 to 8carbon atoms or alkoxy having 1 to 8 carbon atoms,

--X-- is --O--, --S-- or --NH--,

k and m, independently of one another, are 1, 2 or 3, and

t is zero or 1.

Ionophoric compounds, as presented in EP-A-0 451 821, are alsoparticularly suitable for increasing the contrast in the display byrendering of the alignment layer amphiphilic. The ionophores are definedin greater detail by the formula X: ##STR16## where R¹, R², R³ and R⁴,independently of one another, are alkyl having 1 to 15 carbon atoms inwhich one --CH₂ -- group may be replaced by --COO-- or --CO-- or a CH₂-- group which is not bonded directly to the nitrogen atom may bereplaced by --O--, or are cyclohexyl, phenyl or benzyl, and

X is alkylene having 2 to 9 carbon atoms in which one or twonon-adjacent --CH₂ -- groups may be replaced by --O--, two adjacent CH₂-- groups may be replaced by 1,2-phenylene or 1,2-cyclohexylene, twoadjacent --CH₂ -- groups may be replaced by --CH(CH₃)--CH(CH₃)--, andone hydrogen atom of a CH₂ group may be substituted by R⁵ or R⁶, whereR⁵ is alkyl having 1 to 15 carbon atoms and R⁶ is alkyl having 1 to 15carbon atoms or --CH₂ --O--CH₂ --CO--NR¹ R².

Very generally, said compounds can be coupled to or in the alignmentlayer in, for example, the following ways:

I Chemical coupling--i.e. the compound providing a amphiphilic effect ispreferably bonded to/in the alignment layer via covalent bonds. Thecompound to be bonded on has the formula

    C.sub.y --G--R.sub.g

where

C_(y) is one of the abovementioned compounds providing a amphiphiliceffect;

G is a straight-chain or branched alkylene unit having 0 to 18 carbonatoms in which, in addition, one or more --CH₂ -- groups may be replacedby ##STR17## cycloalkanediyl, arenediyl or heteroarenediyl in which, inaddition, one or more hydrogen atoms of the CH₂ groups may be replacedby F;

R_(g) is a reactive group (coupling functionality), for example --OH,--CO₂ H, --CO₂ R, --NH₂, --NHR', --SH, ##STR18## --CH═CH₂, --Si(CH₃)₂Cl, --Si(CH₃)₂ OR', --Si(OR)₃, --N₃, halide, --N.tbd.C or SO₂ CH═CH₂.

Preference is given to compounds in which C_(y) is the macrocycliccompounds, cryptands or coronands described at the outset.

Very particular preference is given to compounds in which C_(y) has thefollowing meaning: ##STR19##

Most preference is given to compounds in which C_(y) is ##STR20## G is--O--(CH₂)_(m) -- or --(CH₂)_(m) --, Y is --O-- or N-alkyl or N-aryl,

R_(g) is --CO₂ R', --N═C═O, --Si(CH₃)₂ OR', --NH₂ or --OH.

II Physisorption

The compounds providing the amphiphilic effect are adducted onto thesurface of the alignment-layer molecules by relatively weak orrelatively strong intermolecular attractive forces. The strength of thecoupling to the surface can be increased by binding polar or polarizablegroups into the compounds providing the amphiphilic effect.

The positive effect of the compounds providing a amphiphilic effect onthe alignment layer can be further reinforced by the liquid-crystalmixtures which likewise contain these compounds, in particular coronandsand cryptands.

In displays, the alignment layer treated according to the invention hasthe effect, in particular, of suppressing twist states and ghost imagesand thus of improving the optical contrast.

Furthermore, the alignment layer rendered amphiphilic can be used toproduce a shock-resistant liquid-crystal switching and display device.Addition of the compounds to be employed according to the invention, inparticular coronands and cryptands, to alignment layers allow the FLCmixtures to be brought into a uniform and twist-free bookshelf orquasi-bookshelf alignment by applying a continuous periodic electricvoltage (explanation of terms: Dubal et at., Proc. 6th Intl. Symp. ofElectrets, Oxford, England (1988); Y. Sato et al., Jap. J. Appl. Phys.28 L 483 (1989)).

It has thus been found that the polymers containing the maleimide unitsdescribed can advantageously be employed as alignment layer inliquid-crystal displays which can be operated at high temperatures,preferably from 30° to 70° C., in particular from 40° to 60° C., asoccur, for example, in projection applications.

Shock-damaged liquid-crystal displays can be re-generated using thesubstances providing an amphiphilic effect in alignment layers byapplying a continuous, periodic voltage, as has already also beenproposed in EP-A 0 451 820 using the substances in FLC mixtures.

The invention is described in greater detail by the examples.

EXAMPLES A) Synthesis Examples Example 1

Preparation of Maleimide-Styrene Copolymer from Maleimide and Styrene

52 g of styrene and 48.5 g of maleimide, which are commerciallyavailable, are dissolved in 350 ml of cyclohexanone and warmed to 80° C.under a stream of nitrogen. After 0.5 g of AIBN has been added, themixture is stirred at 80° for a further 4 hours. The resultantcomposition is diluted with cyclohexanone and filtered through apressure filter. The polymer, whose molecular weight is between 10,000and 40,000 (M_(w), determined by GPC), is precipitated by addition ofmethanol.

Yield: 96% ##STR21## Polymers P2-P6 are prepared analogously.

Example 2 ##STR22## Example 3 ##STR23## Example 4 ##STR24## Example 5##STR25## Example 6 ##STR26## Example 7

Preparation of Maleimide-Styrene Copolymer from Maleic Anhydride-StyreneCopolymer

3.0 g of maleic anhydride-styrene copolymer, styrene content 50%, M_(w)350,000, are heated at 150° C. for 18 hours in a shaken autoclavetogether with 40 ml of 25% strength by weight aqueous NH₃ ; a pressureof 16 bar becomes established.

After cooling, the majority of the polymer is in opaque solution. Thereaction product is evaporated in a rotary evaporator, first atatmospheric pressure and subsequently in vacuo. The solution has a hightendency to foam. The flask contents which remain are removed, dried at80° C. in vacuo, comminuted and dried further to constant weight, giving2.8 g of a pale powder; elemental analysis gives 6.0% N, calculated 6.9%N. The infra-red spectrum (DMSO) shows a broad carbonyl-imide band at1630-1700 cm⁻¹.

Example 8

N-Hexylmaleimide-Styrene Copolymer

5 g of maleic anhydride-styrene copolymer having a styrene content of 50mol %, M_(w) 350,000, is refluxed for 18 hours with 2.5 g of hexylamine(=3.3 ml) in 100 ml of toluene, where the toluene runs out over aSoxleth [sic] apparatus filled with CaCl₂ as desiccate and mounted ontop of the apparatus, and is thus kept dry. The toluene is removed byvacuum distillation in a rotary evaporator, and the flask contents aresubsequently freed from any residues of hexylemine in a high vacuum. Theresidue is dried in vacuo over conc. H₂ SO₄, giving about 7 g of polymerhaving an N content of 4.5%, calculated 4.8%. The infra-red spectrumshows a C═O band at 1650-1700 cm⁻¹.

Further amines are reacted with a maleic anhydride-styrene copolymeranalogously to the processes described in Examples 1 and 2. Sincetemperatures of above 110° C., better >150° C., are necessary for thereaction, a pressure vessel must be used for low-boiling amines (Example7). In the case of higher-boiling amines, the process can be carried outunder reflux in toluene, with the water liberated during the reactionbeing removed, for example as described in Example 8.

Table A below shows amines as reactants which are reacted with theabove-mentioned copolymer (M_(w) 350,000, 50 mol % of styrene) intoluene by the methods of Examples 7 or 8. Also shown are the reactiontemperature and duration and the nitrogen content determined byelemental analysis, if appropriate also the fluorine content. Forwork-up, the polymer product precipitated at low temperature is filteredoff via a suction filter, the precipitate is washed with methanol andthe precipitate is dried in vacuo for 48 hours over conc. H₂ SO₄ toconstant weight. The conversion to the N-alkylimide compound can bemonitored in the IR through the disappearance of the anhydride and at1810/1750 cm⁻¹ and the appearance of the imide carbonyl band at 1700cm⁻¹.

                                      TABLE A                                     __________________________________________________________________________                                Molor ratio                                                                   with respect                                      Example                     to maleic   Temp.                                                                             Reaction                                                                            Yield                       No.  Amine                  anhydride                                                                            Method                                                                             °C.                                                                        duration h                                                                          %                           __________________________________________________________________________    9    CH.sub.3 NH.sub.2      1:1    Ex. 7                                                                              .sup. 150°                                                                 18    >90                         10   (CH.sub.3).sub.3 CNH.sub.2                                                                           1:1    "    "   "     "                           11   n-C.sub.12 H.sub.25 NH.sub.2                                                                         1:1    Ex. 8                                                                              110 "     "                           12   n-C.sub.18 H.sub.37 NH.sub.2                                                                         1:2    "    "   "     "                           13   n-C.sub.18 H.sub.37 NH.sub.2                                                                         1:1    "    180 "     "                           14                                                                                  ##STR27##             2:5    "    110 "     "                           15   HOCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 NH.sub.2                                                      1:1    "    "   "     "                           16                                                                                  ##STR28##             1:1    Ex. 8                                                                              110 18    >90                              Phenylalanine methyl ester                                               17   C.sub.8 F.sub.17C.sub.2 H.sub.21OC.sub.3 H.sub.6NH2                                                  1:1    "    "   "     "                           __________________________________________________________________________

Example 18

N-Perfluorooctanoylmaleimide-Styrene Copolymer

2.0 g of maleimide-styrene copolymer are dissolved in 150 ml ofrefluxing THF. 0.3 g of 80% NaH is added in portions with stirring undera protective gas, and the mixture is stirred at 70° for a further 30minutes. The sodium imide compound prepared in this way is reacted with2.25 ml (=4.32 g) of perfluorooctanoyl chloride, vigorous foamingoccurring initially. The batch is kept at 70° for 4 hours and cooled toroom temperature, after which a precipitate forms which is filtered offwith suction and washed with methanol. Drying in a high vacuum gives 3.8g of a white polymer. According to elemental analysis, this has afluorine content of 30.8% compared with a calculated fluorine content of47.7% after complete conversion, i.e. about 2/3 of the imide nitrogen isoccupied by perfluorooctanoic acid.

Example 19

N-(Butoxycarbonylethyl)Maleimide-Styrene Copolymer

2 g of maleimide-styrene copolymer are dissolved in 25 ml of DMF, 3 g oftriphenylphosphine and 1.8 ml of ethyl azodicarboxylate are added, andthe mixture is stirred for 15 minutes, during which slight warmingoccurs. 1.5 ml of L-(-)-butyl lactate are then added, and the mixture isstirred at room temperature for 20 hours, giving a clear, red-brownsolution, which is poured into 300 ml of methanol. A milky-whiteprecipitation of the polymer is obtained. Since the substance has atacky consistency, it is separated off by centrifugation. After repeatedwashing with methanol and centrifugation, the product is dried at 70° C.in a drying cabinet. Yield 2.5 g. The polymer exhibits a rotation valueof [α]_(D) ²⁰ =+6.7° in the DMF solution.

Example 20

Modification of a Maleic Anhydride-Styrene Copolymer Alignment Layer byMeans of Dodecylamine

2 sheets of glass prepared for cell construction and coated withindium-tin oxide (ITO) electrodes are coated, as described above, byspin-coating with a maleic anhydride-styrene copolymer which has anM_(w) of 50,000 and a styrene content of 50 mol %. The solution is 0.5%strength by weight in methoxypropanol. After heating and fixing, a 1%strength by weight solution of dodecylamine in xylene is applied to thecopolymer by spin-coating in a further operation. The lysine is fixed tothe alignment layer by heating for a further 30 minutes at 130° C.Cooling is followed by washing twice with isopropyl alcohol and thendrying. The glass sheets prepared in this way are then bonded in themanner described below to give test cells.

Particular Embodiments

In a particular embodiment, the novel alignment layer P1(maleimide-styrene copolymer) is modified at the surface by means of anadditive Z1. Z1 contains functional groups which enable chemical bondingto the polymer P1. ##STR29## The bonding of the additive Z1 to thealignment layers P2 to P5 is carried out analogously.

In a further preferred embodiment, the novel alignment layer P1(maleimide-styrene copolymer) is modified at the surface by means of anadditive Z2. Z2 contains functional groups which enable chemical bondingto the polymer. ##STR30##

B) Measurement Examples

Construction of Test Cells

In order to demonstrate the advantageous properties of the alignmentlayers according to the invention, test cells are produced, filled withferroelectric liquid-crystal mixtures and then tested.

To this end, glass plates coated with indium-tin oxide (ITO) are cut andtreated photolithographically so that small glass substrates having anelectrode area of about 8×8 mm² are formed. These glass substrates arethen cleaned first in an aqueous surfactant solution and subsequentlytwice in Millipore water (=demineralized water which has beensubstantially freed from particles via a Millipore filter unit) at about60° C. in an ultrasound bath. After the glass substrates have been driedby means of hot air, they are coated with a wet film of a 0.5% strengthby weight solution of P1 in cyclohexanone. The coating is carried out bymeans of a spin coater, but it can also be carried out by means of othermethods, for example printing or immersion. The solution is dripped ontothe glass substrate until it covers the latter completely and ispre-spun for 5 seconds at 500 rpm, after which the main spinning iscarried out for 30 seconds at 4000 rpm. The wet film is dried at 160° C.for 30 minutes. The remaining layer thickness of P1 is approximately 15nm. This alignment layer is then rubbed with a velvet-like material on arubbing machine (bench speed: 100 mm/min; roller speed 500 rpm; powerconsumption: 0.4/A). The 1.8 μm spacers are then applied by means of thespin coater (0.05% strength by weight solution in isopropanol; 20 sec,2000 rpm). The adhesive frame is printed by means of a plotter, and theliquid-crystal test cells are then bonded with the rubbing directionsparallel using a membrane press (adhesive conditions: Epoxy 304 (5parts)+curing agent 310 B (1 part) (both from E.H.C. Japan). Ethylacetate is then added to the mixture in a ratio of 1:4. Curingtemperatures: 20 min/60° C., 20 min/90° C., 40 min/150° C.).

The resultant test cells are investigated with respect to theirelectro-optical characteristics by means of various liquid-crystalmixtures.

In a further embodiment, an addition layer Z1 is applied in addition tothe aligrunent layer P1 according to the invention. To this end, a 0.5%strength by weight solution of Z1 in 1,4-dioxane is spin-coated onto thealignment layer at 500 rpm for 5 seconds and at 3000 rpm for 30 seconds.This wet film is then dried at 120° C. for 30 minutes, giving chemicalfixing of the additive. The excess additive is then, after the drying,washed off with 2-propanol in an ultrasound bath (1 min). After thisstep, the test cell is again constructed in the manner described above(rubbing etc).

The alignment layers P2 to P6 and the addition layer Z2 are processedanalogously.

In order to characterize the alignment layers according to theinvention, ferroelectric liquid-crystal mixtures are used. The followingare assessed: the alignment of liquid crystal in the test cellcontaining the alignment layer according to the invention; the switchingbehavior of the liquid crystal on application of short addressingpulses; the switching behavior of the liquid crystal on application ofaddressing pulse sequences which simulate operation of a matrix display;and the optical contrast, which is the ratio between the transmissionsin the bright and dark switching states.

The alignment layers according to the invention are used both in thechevron structure described at the outset and in the bookshelf orquasi-bookshelf structure. The bookshelf structure is induced from thechevron structure by application of a rectangular voltage of about 10 Hzat an amplitude of from about 10 to 15 V/μm. The cells are positioned inthe ray path of a polarizing microscope to which, in addition, aphotodiode is attached. The photodiode is connected to a storageoscilloscope and enables recording of the optical transmission of theliquid-crystal cell.

A freely programmable function generator with subsequent voltageamplifier provides the switching pulses necessary for switching to thetest cell. A variety of pulse shapes can be input into the functiongenerator via a computer interface.

The pulse shape used simulates matrix-display operation of the 1-pixeltest cells employed. The line voltage/column voltage ratio (data pulses)is an important parameter which is defined as the bias ratio. This ratioshould be as large as possible, since correspondingly high contrast isonly possible for a low data-pulse amplitude.

The contrast is determined via the signals from the photodiode as theratio between the bright and dark transmissions. A distinction canfurther be made between the contrast of the memory states (without datapulses) and the contrast in the matrix display (with data pulses), thelatter always having lower values.

A further important parameter used to characterize the alignment layersis the effective tilt angle. Twice the tilt angle is identical to theswitching angle. The effective tilt angle in the chevron structure issmaller than the molecular tilt angle (inclination of the molecules tothe layer perpendiculars) as a consequence of the angled layerstructure.

In addition, the occurrence of twist or bend states results in a furtherreduction in the effective tilt angle. The bluish color observed herefor the dark switching state and the low transmission in the brightstate result in low contrast.

The effective tilt angle in the bookshelf structure is significantlylarger, and the bright state is thus distinguished by greaterbrightness. However, the occurrence of twist states again results inconsiderable losses in contrast. The alignment layers employed shouldtherefore substantially suppress the formation of twist states. Theseproperties are characterized using the tilt angle in the chevronstructure.

The FLC mixture M1 employed has the following composition (in mol %):##STR31## and the phase sequence S_(c) * 65 S_(A) * 73 N* 86 I, with aspontaneous polarization of 38 nC·cm⁻² at a temperature of 25° C.

The FLC mixture M2 employed has the following composition (in mol %)

99.5 mol % of FLC mixture M1

0.5 mol % of ##STR32## and the phase sequence S*_(C) 62 S*_(A) 70 N* 83I, with a spontaneous polarization of 34 nC·cm⁻² at a temperature of 25°C.

The FLC mixture M3 employed had the following composition (in mol %)

99.5 mol % of FLC mixture M1

0.5 mol % of ##STR33## and the phase sequence S*_(C) 63 S*_(A) 69 N* 86I, with a spontaneous polarization of 33 nC·cm⁻² at 25° C.

The effective tilt angle and the alignment of the liquid crystal areassessed in the chevron structure, which is formed immediately after thecells are filled. By contrast, the multiplexing and switching propertiesare measured in the bookshelf or quasi-bookshelf structure, which isobtained from the chevron structure by applying rectangular voltages(10-15 V/μm at 10 Hz, 30 s).

The examples below are measured at room temperature.

    ______________________________________                                        Example 21    Test cells containing the novel                                               alignment layer P1 are filled with                                            ferroelectric liquid-crystal mix-                                             tures M1 and M2. The results are                                              show in Table 1.                                                Example 22    In a particular embodiment, P1 is                                             reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 2.                                                        Example 23    In a further embodiment, P1 is                                                reacted with additive Z2 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 2.                                                        Example 24    In a further embodiment, P3 is                                                reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 2.                                                        Example 25    Test cells containing the novel                                               alignment layer P2 are filled with                                            ferroelectric liquid-crystal mix-                                             tures M1 and M2. The results are                                              shown in Table 3.                                               Example 26    In a further embodiment, P2 is                                                reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 3.                                                        Example 27    Test cells containing the novel                                               alignment layer P4 are filled with                                            ferroelectric liquid-crystal mix-                                             tures M1 and M2. The results are                                              shown in Table 4.                                               Example 28    In a further embodiment, P4 is                                                reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 4.                                                        Example 29    Test cells containing the novel                                               alignment layer P5 are filled with                                            ferroelectric liquid-crystal mix-                                             tures M1 and M2. The results are                                              shown in Table 5.                                               Example 30    In a further use form, P5 is                                                  reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are likewise                                           shown in Table 5.                                               Example 31    Test cells containing the novel                                               alignment layer P6 are filled with                                            ferroelectric liquid-crystal mix-                                             tures M1 and M2. The results are                                              shown in Table 6.                                               Example 32    In a further use form, P6 is                                                  reacted with additive Z1 as                                                   described above. The cells contain-                                           ing the resultant alignment layer                                             are likewise filled with mixtures                                             M1 and M2. The results are shown in                                           Table 6.                                                        Reference Example 33                                                                        As reference example, liquid-crystal                                          cells from E.H.C. Japan                                                       (Tokyo), which contain a polyamide                                            (PIX 1400 Hitachi) as alignment                                               layer, are used. The cells are                                                likewise filled with mixtures M1                                              and M2.                                                         ______________________________________                                    

Tables 1 to 6 clearly show the advantages of the novel alignment layersP1 to P6 and, in a particular embodiment, with bonded additive Z1 or Z2.

The novel layers suppress twist states, enable good alignment of theliquid crystal and result in higher effective tilt angles thancomparable polyimide layers. The switching behavior under matrix displayconditions is also significantly better (see maximum bias) compared withthe reference example.

    ______________________________________                                        Example 34    The cells already used in Example                                             22 (P1 with additive Z1 as align-                                             ment layer) are filled with mixture                                           M3. The structure conversion takes                                            place at a temperature of 50° C. with                                  a rectangular voltage of 15 V/μm                                           and 10 Hz for 30 sec. The cell is                                             then left in a memory state for 8                                             days and after this time the                                                  optical contrast in the memory                                                state (CR.sub.man), with multiplex                                            addressing (CR.sub.dyn), the transmission                                     of the bright state in %, relative                                            to parallel polarizers (= 100%),                                              and the CPA (= product of switching                                           pulse height and switching pulse                                              length for switching from 0% to                                               90%) necessary for switching                                                  measured at 30° C., 40° C. and 50° C.      Reference Example 35                                                                        The test cells used utilize a                                                 polyvinyl alcohol as alignment                                                layer to which additive Z1 has been                                           bonded. The test cells were                                                   likewise filled with M3 and                                                   investigated in the same way as in                                            Example 24. The results of Example                                            34 and Reference Example 35 are                                               shown in Table 7.                                               ______________________________________                                    

Compared to the reference layer, the novel alignment layer usedsignificantly higher contrast values as a consequence of lower residualtransmission in the dark switching state. This lower residualtransmission is attributable to improved stability of the structure,which becomes particularly significant at high temperatures. The novelalignment layer is therefore particularly suitable for projectionapplications (projection displays), since temperatures of up to 60° C.can occur in this operation.

                                      TABLE 1                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P1                                                                      Alignment layer P1 + Z1                                                                    Reference example                           __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     8.5°-9°                                                                   10.5° 7°                                   θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜4  ˜5     --                                          (maximum bias at a                (no window present,                         pulse width of 50 μs)          so not multiplexable)                       Twist states                                                                             none      none         form                                        M2                                                                            Effective tilt angle                                                                     11°                                                                              11°   11°                                  θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜6  ˜7     ˜4                                    (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none      none         form                                        __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                                      TABLE 2                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P1 + Z2                                                                    Alignment layer P3 + Z1                                                                    Reference example                        __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     10°   10.5° 7°                                θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good    very good    very good                                liquid crystal                                                                B*max      ˜5     ˜5     --                                       (maximum bias at a                   (no window present,                      pulse width of 50 μs)             so not multiplexable)                    Twist states                                                                             none         none         form                                     M2                                                                            Effective tilt angle                                                                     11°   11°   8.5-9°                            θeff (Chevron                                                           structure)                                                                    Alignment of the                                                                         very good    very good    very good                                liquid crystal                                                                B*max      ˜7     ˜7     ˜4                                 (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none         none         form                                     __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                                      TABLE 3                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P2                                                                      Alignment layer P2 + Z1                                                                    Reference example                           __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     9° 10°   7°                                   θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜4  ˜5     --                                          (maximum bias at a                (no window present,                         pulse width of 50 μs)          so not multiplexable)                       Twist states                                                                             none      none         form                                        M2                                                                            Effective tilt angle                                                                     11°                                                                              11°   8.5-9°                               θeff (Chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜5  ˜7     ˜4                                    (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none      none         form                                        __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                                      TABLE 4                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P4                                                                      Alignment layer P4 + Z1                                                                    Reference example                           __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     10°                                                                              10.5° 7°                                   θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜4  ˜5     --                                          (maximum bias at a                (no window present,                         pulse width of 50 μs)          so not multiplexable)                       Twist states                                                                             none      none         form                                        M2                                                                            Effective tilt angle                                                                     11°                                                                              11°   8.5-9°                               θeff (Chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜5  ˜7     ˜4                                    (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none      none         form                                        __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                                      TABLE 5                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P5                                                                      Alignment layer P5 + Z1                                                                    Reference example                           __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     9.5°                                                                             10°   7°                                   θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜4  ˜5     --                                          (maximum bias at a                (no window present,                         pulse width of 50 μs)          so not multiplexable)                       Twist states                                                                             none      none         form                                        M2                                                                            Effective tilt angle                                                                     11°                                                                              11°   8.5-9°                               θeff (Chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜5  ˜7     ˜4                                    (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none      none         form                                        __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                                      TABLE 6                                     __________________________________________________________________________    Characterization of the novel alignment layer                                            Alignment layer P6                                                                      Alignment layer P6 + Z1                                                                    Reference example                           __________________________________________________________________________    M1                                                                            Effective tilt angle                                                                     10°                                                                              10.5° 7°                                   θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜4  ˜5     --                                          (maximum bias at a                (no window present,                         pulse width of 50 μs)          so not multiplexable)                       Twist states                                                                             none      none         form                                        M2                                                                            Effective tilt angle                                                                     11°                                                                              11°   8.5-9°                               θeff (chevron                                                           structure)                                                                    Alignment of the                                                                         very good very good    very good                                   liquid crystal                                                                B*max      ˜5  ˜7     ˜4                                    (maximum bias at a                                                            pulse width of 50 μs)                                                      Twist states                                                                             none      none         form                                        __________________________________________________________________________     *Maximum possible ratio between switching pulse height and data pulse         height for matrix addressing using the quasibookshelf structure.         

                  TABLE 7                                                         ______________________________________                                        Results of Examples 34 and 35                                                 M3      Alignment layer P1 + Z1                                                                        Polyvinyl alcohol + Z1                               ______________________________________                                        Temper- 30° C.                                                                         40° C.                                                                          50° C.                                                                       30° C.                                                                       40° C.                                                                       50° C.                      ature                                                                         Cr.sub.man                                                                            85      90       90    35    65    60                                 Cr.sub.dyn                                                                            80      80       80    40    60    55                                 Trans-  90      90       90    80    75    90                                 mission                                                                       (bright)                                                                      Trans-  1.06    1.00     1.00  2.29  1.08  1.50                               mission                                                                       (dark)                                                                        ______________________________________                                    

We claim:
 1. An alignment layer for liquid crystal displays, whichcomprises a polymer containing maleimide units of the formula I##STR34## in which R¹ is hydrogen, an acyclic or cyclic, aliphatic,aromatic or araliphatic radical which is chiral or achiral, can bemonosubstituted or polysubstituted by functional groups and in which oneor more CH₂ groups can be replaced by functional groups.
 2. Thealignment layer as claimed in claim 1, wherein R¹ is hydrogen, anaromatic, aliphatic or araliphatic ring having 6 to 24 carbon atoms or abranched or unbranched aliphatic radical having 1 to 40 carbon atoms,which may also contain chiral centers, in which one or more hydrogenatoms, independently of one another, may be replaced by --OH, --F, --Cl,--Br, --CN, --NR² R³, --COOR², --OR², --OSi(CH₃)₃, --SiR² ₂ R³,--Si(OR³)₂ OR³, --Si(OR²)₂ R³ or --OOC--NR² R³, where R² and R³,independently of one another, are hydrogen or an alkyl radical having 1to 6 carbon atoms, and in which one or more CH₂ groups may be replacedby --O--, --SO₂ --, --CO--, --CONR² --, --CH═CH-- or --C.tbd.C--.
 3. Thealignment layer as claimed in claim 2 wherein R¹ is hydrogen, a branchedor unbranched alkyl radical having 1 to 20 carbon atoms in which one CH₂group may be replaced by --O-- or --CO--, and in which one or morehydrogen atoms may be replaced by fluorine, or is --CH(CH₃)--CH₂--(O--CH₂ --CH₂)_(n) X, --CH₂ --CH₂ --(O--CH₂ --CH₂)_(n) X,--CH₂(CH₃)--CH₂ --(O--CH₂ --CH(CH₃)--)_(n) X or --CH₂ --CH₂ (O--CH₂--CH(CH₃)--)_(n) X, where n=1 to 10 and X=NH₂ or OH, or the ##STR35##groups.
 4. The alignment layer as claimed in claim 1 wherein the radicalR¹ is a zwitterionic group.
 5. The alignment layer as claimed in claim 1comprising a copolymer or higher polymer comprising maleimide units ofthe formula I and units derived from polymerizable, ethylenicallyunsaturated compounds of the formula II ##STR36## in which R⁴ and R⁷ arehydrogen, or aliphatic and/or aromatic radicals which may bemonosubstituted or polysubstituted by functional groups.
 6. Thealignment layer as claimed in claim 6, wherein R⁴ to R⁷ are hydrogen, anaromatic radical having 6 to 10 carbon atoms or a branched or unbranchedalkyl group having 1 to 10 carbon atoms in which one or more nonadjacentCH₂ groups may be replaced by --O--, --OOC--, --COO--, --Si(CH₃)₂ -- or--O--CONR⁸ --, and one or more H atoms may be replaced by --OH, --Cl,--Br, --NO₂, --CN, --COOR⁹, --OR¹⁰, --O--Si(CH₃)₃ or --O--COR¹¹ R¹²,where R⁸, R⁹, R¹⁰, R¹¹ and R¹² are hydrogen or an alkyl radical having 1to 5 carbon atoms.
 7. The alignment layer as claimed in claim 1, whereinthe polymer comprising maleimide units of the formula (I) has beenrendered amphiphilic.
 8. A liquid crystal display comprising a liquidcrystal layer which is enclosed on both sides by layers which are inthis sequence starting from the liquid-crystal layer at least onealignment layer, electrodes and a limiting plate, comprising analignment layer as claimed in claim
 1. 9. The liquid crystal display asclaimed in claim 8, wherein the liquid crystal layer is ferroelectric.10. The liquid crystal display as claimed in claim 8, which is operatedat a temperature from 30° to 70° C.